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428
CHEMICAL ENGINEERING
icross S. The scale of the instrument is graduated to read the potential drop )ver S and since this potential difference is made equal to the electromotive i'orce of the couple by the initial setting for zero deflection, the galvanometer indicates directly the true electromotive force of the couple. The initial setting is not altered by introducing resistance into the thermocouple circuit so that the instrument is really a form of potentiometer. The scale may be graduated to indicate temperature for any particular type of couple and the instrument may be obtained with several different scale ranges. The Northrup continuously deflecting pyrovolter is the ordinary pyrovolter with the addition of an extra key and an adjustable resistance.
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FIG. 7.—Northrup pyrovolter.
Deflection Potentiometer Method.—A potentiometer is ordinarily used as a null instrument. The electromotive force of the couple is exactly balanced by the potential drop over a resistance through which a current from a battery is flowing. No current flows through the couple circuit when the proper setting is obtained. This condition is indicated by zero deflection of the galvanometer in series with the couple. The objection sometimes raised against the ordinary potentiometer for industrial installations is that it requires a manual adjustment of a dial or slide wire every time an observation is made. This objection is practically eliminated in the deflection potentiometer which may be constructed so that it embodies the accuracy of the ordinary potentiometer and the convenience of the galvanometer indicator.
In the deflection potentiometer part of the electromotive force of the couple is balanced against the potential drop over a resistance through which a current is flowing and the remaining portion of the electromotive force is indicated by the deflection of the galvanometer in series with the couple. For example, the instrument may be constructed with a dial of say 16 points representing potential differences from 0 to 15 millivolts and a galvanometer which gives full scale deflection on 1 millivolt. The dial is set to the approximate electromotive force developed by the couple and the dial reading combined with the galvanometer reading gives the true electromotive force of the couple. If the couple developed an electromotive force of 13.58 millivolts, the value 13 is obtained from the dial and the value 0.58 is read from the galvanometer scale. Usually the scale is graduated both ways with zero at the center so that + or — deflections may be readily shown. Thus in the above example, if the dial had been